1. Field of the Invention
This invention relates to sensor assemblies for detecting information about an event, particularly assemblies for distinguishing between multiple events.
2. Introduction to the Invention
Sensor cables and assemblies comprising them are well-known. Such sensors may be used to detect changes in variables along an elongate path, e.g. the presence of a liquid such as water or an organic solvent, the attainment of a predetermined temperature or pressure, the presence or absence of light or another form of electromagnetic radiation or a change in the physical position of a movable member, e.g. a valve in a chemical process plant or a window in a building fitted with a burglar alarm system. Changes of this kind are referred to in this specification by the generic term "event". Reference may be made, for example, to U.S. Pat. Nos. 1,084,910, 2,581,213, 2,691,134, 3,248,646, 3,382,493, 3,564,526, 3,470,340, 3,800,216, 3,991,413, 4,278,931, 4,400,663 and 4,570,477, U.K. Pat. Nos. 182,339, 1,355,176 and 1,481,850, German Offenlegungschriften Nos. 3,011,500 and 3,225,742, European Patent Application Publication Nos. 133,748, 144,211, 160,440, 160,441, and 164,838, and copending, commonly assigned application Ser. Nos. 838,725 published as International Patent Application No. WO 86/07483 (Lahlouh et al.) and 832,662 (Masia et al.) now abandoned in favor of a continuation application, Ser. No. 306,237, filed Feb. 7, 1989. The disclosures of each of the patents, applications, and other publications referred to above are incorporated herein by reference.
Sensors such as those described in the above publications are useful for detecting leaks from pipe lines, leaks from steam lines into thermal insulation surrounding such lines, and leaks from tanks containing corrosive or noxious chemicals. Such sensors are relatively slow to respond to the presence of a solvent, may be sensitive to flexing, pressure, or other mechanical abuse, and are subject to "nuisance tripping" in the presence of solvent vapors. An additional problem occurs when more than one liquid is present. For example, sensing means capable of distinguishing between a conductive liquid, e.g. water, and a nonconductive liquid, e.g. gasoline, are often required between the inner and outer walls of a dual wall containment tank. Conventional sensing apparatus is not designed for this purpose. Rather, several sensing wires, one pair for each liquid or event to be detected, must be used, eliminating flexibility and requiring complex electronics.